A wide variety of iron-sulfur clusters and related mixed-metal species are found as active sites in metalloenzymes. In some cases these proteins form part of electron transport chains., and in other cases they serve as catalytic centers for quite unusual chemistry. Our research focuses on the use of modern techniques for quantum chemistry to carry out theoretical studies (at the spin Hamiltonian and density functional level) of their electronic structures, in an attempt to more closely connect the spectroscopy of these systems to their structure and function. Two important areas will be emphasized during the next period of this project. The first will be the nitrogenase system, catalyzing the reduction of dinitrogen to ammonia. Detailed quantum mechanical models will be prepared for the "P" and "M" clusters in the molybdenum-iron protein, and also for the iron-sulfur cluster in the iron protein. The second area will involve similar studies on sulfite reductase, which catalyzes the reduction of sulfite to hydrogen sulfide, and one mechanistically similar nitrite reductases, which promotes conversion of nitrite to ammonia. Overall, this work aims to create a theoretical and conceptual framework that supports integration of many types of spectroscopic and energetic data in a way that is helpful in understanding biochemical function. A key technical advance will involve the development of methods to include the effects of the protein and solvent environment into quantum chemistry models of the metal-centered active sites, allowing a more direct comparison to redox and protonation energetics, and to improved interpretation of spectroscopic parameters.